Injection molded resin synthetic articles having a dull surface are generally produced by injection molding with metal molds having surfaces with fine asperities. However, several defects have generally been pointed out to exist in the injection moldings obtained by this method, such as the failure to reproduce details of the fine asperities from the mold surfaces and/or noticeable unseemly spots such as weld lines and flow marks and there is a need to make improvements.
One of the practices being employed is to improve these defects in appearance by adopting special molding conditions. While there are a variety of molding conditions to adopt, the greatest effect is exhibited by the mold temperature and the higher the mold temperature, the more preferred. However, at higher mold temperatures, the cooling time required to cool and solidify the plasticized resin is prolonged to lower the molding efficiency.
Under the circumstances, a method is required that achieves good reproduction of the die surfaces without increasing the mold temperature or that permits the mold temperature to be increased without prolonging the required cooling time.
In another method being practiced, a heating and a cooling port are provided in a mold and a heating and a cooling medium are flowed alternately to repeat the heating and cooling of the mold; this method, however, consumes much heat and requires a prolonged cooling time.
A method of providing a better reproduction of mold surfaces by coating the mold cavity defining inner die surfaces with a material having a small heat conductivity is disclosed in the specification of U.S. Pat. No. 3,544,518 with respect to injection molding.
In addition, the specification of U.S. Pat. No. 4,919,388 describes a metal mold having the mold cavity defining inner die surface coated with a heat insulating layer having surface asperities. However, little mention is made of the shape of the surface asperities of the heat insulating layer, the state of the surfaces of the shaped article, the operational and advantageous aspects of the layer, etc.
The object of the present invention is to provide an injection molded article that has a uniform dull surface with a less noticeable weld line and other surface detects and that can be produced with improved efficiency, as well as a method for molding such articles.
The mold surfaces that define the die cavity of a metallic mold have heretofore generally been rendered dull exclusively by a method that roughens the mold surfaces by sandblasting. With molds having the coating of a heat insulating layer, sandblasting may be the first method to try to roughen the die surface of the heat insulating layer to make them dull. We performed injection molding of synthetic resins using an insulating layer coated mold that had the surface sandblasted to a dull state; as a result, it was found that despite the uniform dull state of the die surface, the injection molded articles were not uniformly dull. Stated more specifically, there were not any unseemly weld line dents which would have occurred if molding had been done with common dull-surfaced metal molds; on the other hand, it was found that peculiar defective phenomena occurred noticeably in that the weld and general portions of the injection molded article and/or the general and resin flow end portions produced non uniform dull surfaces and that, in addition, the dull surface of the weld or resin flow end portion would readily sustain a flaw when scratched by nail.
The present invention provides an injection molded article having a superior appearance in that it has accomplished various features such as a less noticeable weld line, uniform dull surface and improved abrasion resistance, as well as a method of molding said article.